docs: clarify wording about how to use advanced glance image
docs: clarify wording about how to use advanced glance image

This is to follow up on comments in
https://review.openstack.org/#/c/469838/

Change-Id: I59a0f63e56d972a2fadf5a4e0bf531ccf4da8dba

noironetworks/neutron,openstack/neutron,mahak/neutron,eayunstack/neutron,openstack/neutron,noironetworks/neutron,mahak/neutron,eayunstack/neutron,huntxu/neutron,huntxu/neutron,mahak/neutron,openstack/neutron